Electrostatic resonators are known, in particular from documents EP-A-0 810 418 or FR-A-2 792 722, comprising a vibrating member in the form of a metallized bell adapted to be set into vibration at a resonant frequency under the effect of electrostatic forces generated by electrodes disposed facing a portion of the vibrating member.
The resonator is adapted to operate in rate gyro mode or in free gyro mode. In rate gyro mode, the vibrating member is excited by means of a combination of control signals applied at the resonant frequency of the vibrating member and modulated to generate an amplitude control signal, a precession control signal, and a quadrature control signal, these control signals being applied in such a manner that measuring the vibration of the vibrating member and demodulating said vibration at the resonant frequency of the vibrating member enable the speed of rotation to which the resonator is being subjected to be determined by making use of appropriate equations.
The accuracy with which the speed of rotation is calculated is naturally a function of the accuracy with which the various terms of the equation giving the speed of rotation to be measured are themselves determined. One of these terms is the amplitude of the vibration obtained by applying amplitude control. It is this term which is known with the least accuracy and stability. Furthermore, quadrature control is applied in the same directions as precession control, and when the quadrature control and the precession control are both applied in the form of a signal modulated at the resonant frequency, any phase error in implementing quadrature control becomes projected onto precession control and gives rise to resonator drift error.
The same problems arise when the resonator is used in free gyro mode, i.e. when the vibrating member is excited by means of a combination of control signals comprising only an amplitude control signal and a quadrature control signal.
It is also known from document U.S. Pat. No. 5,850,041 to control a resonator in free gyro mode by applying the amplitude control at twice the resonant frequency and by applying quadrature control in the form of a DC voltage. However, according to that document, quadrature control is applied separately from amplitude control over a very large number of separate electrodes, i.e. sixteen or even thirty-two separate electrodes. The resonator is thus itself extremely expensive to make, not only because of the difficulty of making a large number of electrodes accurately, but also because of the difficulty of making connections between all of those electrodes and an external processor. In addition, the associated control circuit is very complex and thus also very expensive.